neuter

Definitions

from The American Heritage® Dictionary of the English Language, 4th Edition

adj. Grammar Neither masculine nor feminine in gender.

adj. Grammar Neither active nor passive; intransitive. Used of verbs.

adj. Biology Having undeveloped or imperfectly developed sexual organs: the neuter caste in social insects.

adj. Botany Having no pistils or stamens; asexual.

adj. Zoology Sexually undeveloped.

adj. Taking no side; neutral.

n. Grammar The neuter gender.

n. Grammar A neuter word.

n. Grammar A neuter noun.

n. A castrated animal.

n. A sexually undeveloped or imperfectly developed insect, such as a worker bee.

n. A plant without stamens or pistils.

n. One that is neutral.

transitive v. To castrate or spay.

from Wiktionary, Creative Commons Attribution/Share-Alike License

adj. Neither the one thing nor the other; on neither side; impartial; neutral.

adj. Having a form belonging more especially to words which are not appellations of males or females; expressing or designating that which is of neither sex.

adj. Intransitive

adj. Having no generative organs, or imperfectly developed ones; sexless.

n. The neuter gender.

n. A noun of the neuter gender; any one of those words which have the terminations usually found in neuter words.

n. An organism, either vegetable or animal, which at its maturity has no generative organs, or but imperfectly developed ones, as a plant without stamens or pistils, as the garden Hydrangea; especially, one of the imperfectly developed females of certain social insects, as of the ant and the common honeybee, which perform the labors of the community, and are called workers.

n. A person who takes no part in a contest; someone remaining neutral.

n. An intransitive verb or state-of-being verb.

v. To remove sex organs from an animal to prevent it from having offspring; to castrate or spay, particularly as applied to domestic animals.

v. To rid of sexuality

from the GNU version of the Collaborative International Dictionary of English

adj. Neither the one thing nor the other; on neither side; impartial; neutral.

adj.

adj. Having a form belonging more especially to words which are not appellations of males or females; expressing or designating that which is of neither sex

adj. Intransitive.

adj. Having no generative organs, or imperfectly developed ones; sexless. See Neuter, n., 3.

n. A person who takes no part in a contest; one who is either indifferent to a cause or forbears to interfere; a neutral.

n.

n. A noun of the neuter gender; any one of those words which have the terminations usually found in neuter words.

n. An intransitive verb.

n. An organism, either vegetable or animal, which at its maturity has no generative organs, or but imperfectly developed ones, as a plant without stamens or pistils, as the garden Hydrangea; esp., one of the imperfectly developed females of certain social insects, as of the ant and the common honeybee, which perform the labors of the community, and are called workers.

transitive v. To render incapable of sexual reproduction; to remove or alter the sexual organs so as to make infertile; to alter; to fix; to desex; -- in male animals, to castrate; in female animals, to spay.

from The Century Dictionary and Cyclopedia

Neither the one thing nor the other; not adhering to either party; taking no part with either side, as in a contention or discussion; neutral.

In gram.:

Of neither gender; neither masculine nor feminine: used when words are grammatically or formally distinguished as masculine, feminine, and neuter—a distinction made in English only in the pronouns he, she, it.

Neither active nor passive; intransitive. Abbreviated n. and neuter

In botany, same as neutral.

In zoology, having no fully developed sex: as, neuter bees.

n. A neutral.

n. An animal of neither sex, and incapable of propagation; one of the imperfectly developed females of certain social insects, as ants and bees, which perform all the labors of the community; a worker. See cuts under bee, Atta, and Termes.

n. In botany, a plant which has neither stamens nor pistils. See cut under neutral.

Regarding the use of “millenniums,” it’s also a feature of language that irregularities regularize, and since we have almost no one left anymore who knows Latin, it’s to be expected that we will not retain Latin neuter plural endings in lieu of the predominate English “s” plural pattern.